The present invention relates to an extruder. More particularly this invention concerns a drive for an extruder.
A standard extruder, for instance used to fill molds with a plastified thermoplastic resin, has a basically tubular housing coaxially receiving a screw. Rotation of this screw simultaneously displaces and plastifies plastic granules fed radially into an upstream portion of the housing and exiting as a hot fluent mass axially from a downstream end of the housing. The screw is rotated at a fairly low speed by a drive typically comprised of a motor whose output shaft is connected through a transmission to the upstream end of the extruder screw.
Thus in the standard system the extruder and its drive are a fairly long and massive assembly. When several extruders must be used together to make a workpiece formed of several different resins in a coextrusion process, it is difficult to find room for the necessary extruders and their drives. Furthermore the bulky transmissions are noisy and require substantial maintenance.
It is therefore an object of the present invention to provide an improved extruder assembly.
Another object is the provision of such an improved extruder assembly which overcomes the above-given disadvantages, that is which has a substantially simplified drive.
An extruder has a housing and a screw rotatable in the extruder housing about an axis. A drive has according to the invention a drive housing fixed to the extruder housing, an extension fixed directly to the screw and extending along the axis into the drive housing, and a sleeve-shaped rotor centered on the axis in the drive housing and coaxially surrounding the screw extension. Structure rotationally fixes the rotor directly to the screw extension. A sleeve-shaped stator centered on the axis and fixed in the drive housing coaxially surrounds the rotor. The stator and rotor together form a motor that can rotate the screw about the axis.
Thus with this system the entire drive is coaxial with the extruder screw and mounted directly on the extruder housing, so it is possible to crowd several extruders together for a coextrusion process. The motor, which is electric according to the invention, can generate considerable torque even at a low rotation rate. The coaxial rotor and stator are particularly easy to cool, for instance by pumping water through the drive housing around them, for most efficient operation. The rotor is directly connected, that is without a transmission, to the extruder screw, thereby eliminating a source of substantial noise. Without the transmission, the need for a complex oil-lubrication system is also eliminated.
According to the invention the structure includes a coupling sleeve fixed to the rotor and closely surrounding the screw extension. The sleeve and extension have axially extending and interengaging formations that rotationally couple them to each other. These formations are typically angularly equispaced and of complementary shape for good rotational interconnection of the sleeve and extension. Such splining together of these two parts makes it easy to separate them axially from each other for servicing of the extruder.
The structure further comprises a plate extending generally perpendicular to the axis and fixed to the coupling sleeve and to the rotor. Screws or bolts secure this coupling plate with the sleeve and with the rotor. More particularly the plate is annular and has a central hold big enough that the screw and extension can be pulled axially out through it. Thus the drive does not get in the way when the screw has to be replaced or serviced. This makes it possible to draw the screw out of the upstream or rear end of the extruder housing, as is standard so the connection of the extruder to the mold does not have to be undone. A cover is provided that normally closes this hole, and that can in fact help secure the screw axially to the coupling sleeve and rotor.
The extruder housing includes an annular collar projecting axially between the extension and the rotor and having cylindrical or cylindrically stepped inner and outer surfaces centered on the axis. Bearings on the collar support the sleeve and extension rotatably thereon. These bearings are typically roller bearings set up, at least for the sleeve, to resist both axial and radial forces.
The assembly according to the invention further comprises a cover plate removably secured to the drive housing and engaged axially over the stator. The stator is removable from the drive housing when the cover plate is removed.